1. Technical Field
This invention relates to the field of providing high speed digital data services to a digital service subscriber and, more particularly, to so-called asymmetric digital subscriber line (ADSL) services and to a method of reducing near-end crosstalk in discrete multi-tone (DMT) modems located at a central office and at the subscriber's premises for providing ADSL services.
2. Description of the Related Arts
In the field of cable television, cable modem technology is emerging which provides increased bandwidth services to the home. Cable television equipment manufacturers are promoting the upgrading of cable television distribution plant to comprise so-called hybrid optical fiber and coaxial cable (hybrid fiber/coax) facilities. It is anticipated in the cable field that bandwidths to and from the subscriber can be increased so that bi-directional voice and data services may be provided in addition to traditional cable television programming. In the related field of telecommunications, there exists a considerable amount of embedded distribution plant comprising high capacity twisted wire pair cable. Historically, each cable pair was specially loaded with inductance coils at periodic intervals along the path from a serving central office to the subscriber's premises to improve voice telephony performance. The inductance loading countered the effects of the high capacity cable and provided a flat bandwidth on each twisted cable pair at voice frequencies. On the other hand, frequencies higher than voice bandwidth were intentionally attenuated to such a degree that the twisted wire cable pair was unusable for other than a voice channel. When the loading is removed, the twisted cable pair bandwidth improves and becomes more comparable to that of coaxial cable.
An emerging technology in the telecommunications arts that competes with cable modem technology is so-called asymmetric digital subscriber line (ADSL) technology. Referring to FIG. 1 taken from American National Standards Institute standards document T1.413-1995, there is shown a public switched telecommunication network (PSTN) 105 and a digital network (for example, a frame relay, asynchronous transfer mode (ATM), Internet or other digital network) 110 at the left. At the right is the subscriber's premises. The digital network 110 is coupled via a logical interface V to an ADSL transceiver unit (ATU) at the serving central office (C). Also at a serving central office are located a splitter 120 for splitting the telecommunications services from the digital services, typically based on frequency. For example, a voice channel may still be preserved at from 0-4000 Hz. The splitter function may be integrated into ATU-C 115 (and at the remote subscriber site, into ATU-R 135). Interface U-C represents the subscriber loop (twisted pair) interface at the central office C and interface U-R represents the subscriber loop interface at the remote subscriber terminal end of the twisted wire cable pair or other facility 125. Facility 125 may comprise, for example, a twisted wire pair or a hybrid optical fiber/twisted wire pair facility or other wired or wireless facility having comparable or greater bandwidth. Service module (SM) 150 or 155 at the remote location may comprise an intelligent telecommunications terminal, a personal computer, a television terminal, an energy management system, a security system or other service module known in the art. Plain old telephone service (POTS) module 145 represents a traditional telecommunications terminal such as a facsimile terminal, voice bandwidth modem or standard telephone. Facility C1 distribution 140 within the subscriber premises may comprise, for example, a bus such as a home bus or a star network or other configuration. By bus as used herein is intended a communications link that may be wired or wireless connecting a plurality of devices together. The bus may be arranged so that there is contention for access to the bus according to priorities or be provided sufficient capacity to alleviate the likelihood of contention. Interface T represents the interface between a service module (SM) and/or a bus/star 140 to other service modules (SM's).
Referring to FIGS. 2A and 2B, there are shown respectively an ATU-C transmitter whose reference diagram is taken from A.N.S.I. T1.413-1995 and an ATU-C receiver whose reference diagram is derived therefrom. In FIG. 2A, there is shown an expanded functional block diagram of the transmitter portion of ATU-C 115 of FIG. 1. A multiplexer/sync control unit 200 provides the interface to the digital network 110. Various high speed data rate links AS0, AS1, AS2 and AS3 at multiples of 1.536 Mbits/sec are provided toward digital network 110. In particular, each AS link represents an independent downstream simplex (unidirectional downstream) bearer of data traffic. Lower speed data services are also shown and represented by LS0 (16 or 64 kbits/sec), LS1 (160 kbits/sec) and LS2 (384 or 576 kbits/sec). Each LS link may represent a duplex bearer (bi-directional) carrying both downstream and upstream traffic or, in the alternative, a unidirectional simplex bearer.
CRC 205 and CRC 210 represent cyclic redundancy check in each direction of transmission. Scrambler and forward error correction 215, 220 represent scrambling and forward error correction, for example, using Reed-Solomon error correction coding, in each direction of transmission. Interleaver 225 provides a data interleaving function as is further described in A.N.S.I. T1.413-1995, incorporated by reference as necessary. Tone ordering function 230 provides tone selection and control functions as are also described by A.N.S.I. T1.413-1995. Constellation encoder (if used) and gain scaling functions are represented by block 235. The inverse discrete Fourier transform function applied for data modulation is represented by block 240. Two data directions are shown coupling IDFT 240 and output parallel to serial buffer 245 where a cyclic prefix is added to each data frame. Finally, a digital to analog converter and analog signal processing function are represented by block 250 which interfaces the subscriber facility 125.
Referring to FIG. 2B, the ADSL receiver at the central office is shown. The horizontal arrows are reversed in direction from FIG. 2A. Data demultiplexer 255 interfaces the digital network 110. Descrambler 265, 258, deinterleaver 270, decoder 280, DFT 285, input serial to parallel buffer 290 and analog to digital converter 295 represent the significant changes in function between ATU-C transmitter and receiver.
Referring to FIG. 3, at a subscriber terminal, the transmitter (ATU-R) is similarly configured as ATU-C but it is assumed that channels operate at LS0, LS1 or LS2 toward the subscriber's equipment. Cyclic redundancy checks 305/310 are provided for each direction of transmission to/from subscriber equipment 375. Scrambler and forward error correction circuits 315 and 320, for example, using Reed-Solomon error correction coding, are provided for especially secure data transmission. An interleaver 325 is provided in one transmit path. Tone ordering circuitry 330 is necessary for generating and ordering the discrete multi tones of the discret multi-tone (DMT) modem. The constellation encoder and gain scaler 335 may or may not provide a form of trellis data encoding and gain scaling for controlling the tone ordering. IDFT block 340 performs an inverse discrete Fourier transform for modulating the digital data. The output parallel to serial buffer 345 is provided for providing parallel to serial conversion to a digital to analog converter and analog signal processing interface 250 which interfaces the subscriber loop 125.
The analog signal framing (FIG. 4) used in ADSL technology is obtained by passing quadrature amplitude modulation (QAM) samples through a D/A converter 250 or 350. These samples are arranged in a superframe of 69 frames (frames 0-68) totaling approximately 17 milliseconds. Altogether 512 samples (256 real and 256 imaginary, 0-511) are taken of the data. An additional 32 samples contain a cyclic prefix making a total of 544 samples in each frame. The cyclic prefix CP 401 is added, for example, to signal 402 of frame 0 (FIG. 4) at the output parallel/serial buffer 245 and 345 shown in FIGS. 2A and 3 respectively.
Every 69th frame contains a pseudo-random number (PRN) sequence with a nominal length of 544 samples. This PRN sequence (the so-called synch symbol) permits recovery of the frame boundary after interruptions.
The sub-carrier tones are spaced at 4.3125 kHz according to the ANSI Standard T1.413-1995 and at carrier 64 where the frequency is 276 kHz, a pilot carrier is inserted. The data modulated on that pilot is a constant bit value (for example, 0,0). Other details of frame construction, data modulation, tone ordering and the like may be found in the Standard and are not believed to be particularly relevant to the principles of the present invention.
Near-end crosstalk, hereinafter referred to as NEXT, is a potentially severe problem for operating multiple Digital Subscriber Line (DSL) modems over twisted-pair wires between a Central Office (CO) and a subscriber's location. NEXT occurs when the transmissions from one or more modems, particularly those at the central office, capacitively couple into each other's twisted-wire pairs and impair the ability of those modems to receive transmissions from the other end of the twisted-wire pairs. Moreover, with severe NEXT, a subscriber modem cannot receive transmissions from its transmitting central office modem. This problem is most severe when a modem operates in full-duplex mode in which transmission is simultaneously bi-directional at all frequencies.
Cables that serve subscribers and terminate at a central office can comprise thousands of twisted wire pairs that are bundled together in a limited cross-sectional (typically circular) area. Electrical signals traveling on the twisted pairs can easily electrically couple into physically proximate twisted pairs, consequently, near end crosstalk has a detrimental effect on bit error rate. Since the concept behind ADSL technology is to optimize bandwidth use, the phenomenon of near end cross-talk limits any one of three factors: the distance a subscriber can be from the central office, the digital data rate of service and the bit error rate of any digital data service. As the number of subscribers increase to ADSL technology, the likelihood will increase that an ADSL subscriber will be served by a twisted wire cable pair proximate to that of another subscriber and that frequencies from one twisted wire cable pair will adversely impact the signal to noise ratio of digital signals on an adjacent or proximate cable pair. Consequently, there is a need in the art to alleviate the effects of NEXT.
One way to reduce NEXT might be to assign cable pairs to subscribers in such a way that subscribers to ADSL services are not in the same bundles as other subscribers. Of course, at some point in time, as subscribers to ADSL increase, so does the likelihood that cable pair assignment in such a manner cannot be accomplished. Thus, there is a need in the art for reducing NEXT in ADSL services.